The measurement of regional cerebral blood flow requires simultaneous measurement of the xenon clearance rate and the brain-blood partition coefficient. However, in the usual nuclear medicine study only the xenon-133 clearance is measured and the blood flow is calculated using published values of the brain-blood partition coefficient for normal tissue. This may lead to large errors in the case of pathological tissue because the partition coefficient varies greatly in brain tumors. In addition, scatter, poor localization and non-uniform sensitivity with distance from the detector plague the usual studies. The computerized tomographic (CT) scanner is a new imaging device that yields a cross-sectional view of the body with very high sensitivity to slight changes of radiodensity in the various body tissues. We have already shown that the CT scanner may be used to quantitate the amount of stable xenon in the brain. We plan to use this ability to measure the local partition coefficient in vivo. With the animal (and later the patient) inhaling a xenon mixture, one waits for saturation of the brain by xenon. A scan at this time will allow a map of the partition coefficient in the various brain tissues to be produced. Returning the animal (patient) to oxygen inhalation and taking scans during the subsequent washout should allow us to use the local partition coefficient and clearance to produce, for the first time, a perfusion image of the brain. We intend to use this method in adult patients with brain tumors and adult patients with advanced occlusive vascular disease. It is felt that these two groups can justifiably be exposed to the small radiation hazard involved in this method. We expect to obtain accurate measurements for 0.5 cc of brain.